1. Field of the Invention
This invention relates to a thin film SOI wafer having a buried oxide film for forming a high-speed, low power consumption SOI (silicon on insulator) device, and more particularly to a SIMOX (separation by implanted oxygen) wafer in which the buried oxide film is formed by implanting an oxygen ion into a surface of a wafer and then annealing at a high temperature.
2. Description of Related Art
As a method of producing the thin film SOI wafer such as SIMOX wafer, there are known two methods, i.e. a first method is a so-called high-dose SIMOX forming method mainly including a step of implanting a high dose of oxygen ion (16O+), and a second method is a so-called low dose SIMOX forming method including a step of implanting an oxygen ion at a does lower by about one digit than that of the high-dose SIMOX forming method and then annealing in an oxidizing atmosphere having a high oxygen content.
Recently, there is developed a so-called MLD (modified low dose) method wherein a final step of oxygen ion implantation in the low-dose SIMOX forming method is carried out at a low dose in the vicinity of room temperature, which is provided in mass production.
The high-dose SIMOX method is typically a method wherein an oxygen ion (16O+) is implanted under conditions of an acceleration energy: 150 keV, a dose: more than 1.5×1018/cm2 and a substrate temperature: 500° C. and thereafter the annealing is carried out at a temperature of higher than 1300° C. in an argon (Ar) or nitrogen (N2) atmosphere containing 0.5-2% of oxygen for 10-20 hours (see K. Izumi et al. Electron. Lett. (UK) vol. 17(1978), p 593).
Also, the low-dose SIMOX forming method is typically a method for solving problems of the above method, in which the oxygen ion (16O+) is implanted under conditions of an acceleration energy: more than 150 keV, a dose: 4×1017-1×1018/cm2 and a substrate temperature: 400-600° C. and thereafter the annealing inclusive of a high-temperature internal thermal oxidation (which may be abbreviated as ITOX) step is carried out at a temperature of higher than 1300° C. in an oxidizing atmosphere containing 30-60% of oxygen and argon (Ar), whereby the buried oxide film (BOX) can be thickened and also the significant improvement of the quality can be realized (see S. Nakashima et al. Proc. IEEE int. SOI Conf. (1994) p 71-72).
Further, the MLD (modified low dose) forming method is developed as an improving version of the low-dose SIMOX forming method and is a method in which after the oxygen ion implantation step at the conventional high temperature (400-650° C.), an oxygen ion implantation is further conducted at a dose lower than by one digit under room temperature to form an amorphous layer on the surface (see O. W. Holland et al. Appl. Phys. Lett. (USA) vol. 69 (1996), p 574 and U.S. Pat. No. 5,930,643).
There is known that all of these SIMOX forming methods include the high-temperature annealing step and particularly the annealing conditions at the final step thereof largely affect the final quality of the SIMOX wafer. Generally, a heat treatment is carried out at a temperature of higher than 1300° C. in an Ar atmosphere containing about 0.1-2% of oxygen for 10-20 hours at the final annealing step of SIMOX. In this case, it is known that the heat treatment vanishes residual defects, oxygen precipitates and the like in the SOI layer but also grows and unites oxygen precipitates in BOX to vanish defects in BOX, while the roughness at the interface between SOI and BOX is improved to finally improve the surface roughness.
However, the above conventional SIMOX forming methods have a problem that the annealing step at an extremely high temperature for a long time is required for realizing the roughness improving effect (flattening).